1. Field of the Invention
The present invention relates to transitions between a conductor-based transmission line and a three dimensional microwave waveguide.
2. Description of Related Art
In microwave circuit design it is often necessary to interface circuit boards with other circuit components. Circuit boards typically communicate via one of various conductor-based transmission lines, such as microstrip, stripline, coplanar waveguide or slotline. Three-dimensional microwave waveguides typically have rectangular or circular cross sections, and are hollow with metallic shells or are made of waveguide-conducting dielectric. These three dimensional waveguides are referred to herein as microwave waveguides or simply waveguides.
Adaptors or transitions, also referred to as launches, are employed to interface the two different types of media with each other. Such transitions typically suffer from losses due to attenuation and impedance mismatches (reflections). Conventional transitions to microwave waveguide are from stripline or microstrip. The transition is usually via an end of a microwave waveguide section, although it is known to introduce a stripline element laterally through a side of a microwave waveguide, as is illustrated in U.S. Pat. No. 4,716,386 issued to Lait. U.S. Pat. No. 4,901,040 issued to Ahlborn et al. discloses a transition from microwave in which a T-shaped element is positioned in the microwave waveguide.
At very high frequencies, such as above 20GHz, active printed circuits are preferably in the form of coplanar waveguides having a signal conductor bounded by two signal return or ground conductors. Device interconnects are preferably provided by microwave waveguides. The printed circuits allow low cost production while microwave waveguides allow easy interconnections and a low loss transmission line for filters and other components.
There is therefore a need for transitions between conductor-based circuits and microwave waveguides which have a wide transmission bandwidth and have low loss due to the generation of spurious modes. At higher frequencies in which coplanar waveguides are used for printed circuits, it is desirable to have such a transition directly between coplanar waveguide and microwave waveguide.